1. Field of the Invention
This invention is in the field of cryogenic coolers of the split Stirling cycle type. More particularly, this invention relates to a cryogenic cooler with an expander portion of the cold-finger configuration; and having a reciprocable spring/mass system defined in the expander portion of the cooler by the combination of a displacer-drive piston, and a resilience. The spring/mass system includes a combined pneumatic and mechanical spring, and the reciprocable mass includes a drive piston and displacer assembly. The spring/mass system is in near-resonance with the operating cyclic rate of a compressor portion of the cryogenic cooler.
2. Related Technology
Conventional cryogenic coolers of the split Stirling cycle cold finger configuration generally include a compressor alternately supplying and withdrawing pressurize gas, such as high pressure helium gas, to and from an expander portion of the cooler. The expander portion of the cooler defines the "cold finger" portion of the cryogenic cooler, and include a drive part from which extends the elongate finger portion having a distal end portion with an end surface. This distal end portion, and particularly the end surface, is cooled to cryogenic temperatures by operation of the expander. Another portion of the expander near the proximal end of the cold finger is warmed above ambient temperature by operation of the expander. Heat withdrawn from the cooled portion of the expander, as well as heat resulting from mechanical inefficiency of the expander, is rejected to ambient at this warm portion of the expander. These conventional cold finger expanders generally include one of three alternative mechanizations for reciprocally driving the moving mass of the expander in response to the cyclical supply and withdrawal of high pressure gas to and from the expander by the compressor portion of the cryogenic cooler.
One of the conventional mechanizations for such cold finger expanders includes a pneumatic drive piston to drive the displacer in reciprocation. This pneumatic-spring type of drive mechanization essentially employs the conventional pneumatic bounce piston concept to reciprocally drive the displacer. One portion of the pneumatic piston is exposed to the high pressure gas from the compressor, which varies in pressure as the compressor supplies and withdraws this gas. Another portion of the pneumatic piston is exposed to the gas pressure in a substantially closed chamber, which gas pressure also varies as the drive piston reciprocates. The drive piston reciprocates between first and second positions according to the force balance on the portions of the piston, and according to the effective pressures in the closed chamber and as supplied by the compressor.
The conventional displacer drive with a pneumatic piston gives good cryogenic performance, but is usually noisy during cool down from ambient to cryogenic temperatures. This may be the case because the reciprocating mass is not stabilized within the expander, is over driven by the compressor, and impacts the end walls of the expander during cool down. Some cryogenic coolers of this type also continue to be noisy after achieving cryogenic temperature at the cold finger.
An alternative mechanization for the displacer drive of a split Stirling cycle cryogenic cooler includes a spring/mass system with a mechanical spring disposed in the working volume of the expander, and a piston to which the varying gas pressure from the compressor is applied. The spring/mass system is usually arranged to be in near-resonance at the cyclic operating rate of the compressor. These spring-drive coolers are generally quiet both during cool down and after achieving cryogenic temperatures at the cold finger. However, this design of cooler also suffers from an inherently lower efficiency of the expander because of a larger dead volume connecting to the working chamber of the expander. In other words, the expander has a lower compression ratio in conjunction with the compressor. Such is the case because the volume necessary to house the mechanical spring connects to the working chamber of the expander, and lowers the effective compression ratio of the compressor-expander combination. These cryogenic coolers accordingly have a lower efficiency of performance in terms of watts of cooling compared to watts of drive power applied at the compressor. Also, the warmed heat-rejecting portion of the expander has a higher operating temperature than the pneumatic-piston type of expander due in large extent to unusable heat of compression that takes place in the added void volume in which the springs of the expander mechanism are located.
Yet another type of expander drive mechanization for the cold finger of a split Stirling cycle cryogenic cooler includes the use of a pneumatic piston to drive the displacer, in combination with a pair of centering springs to stabilize the center position of reciprocation of the displacer after cool down. Conventionally, this spring-centered type of drive mechanization would not operate at near-resonance with the cyclic operation rate of the compressor, and the centering springs would be isolated from the working volume of the expander. This latter type of expander drive mechanization has good cryogenic performance, which is generally comparable favorably to the pneumatic drive piston expanders. Expanders of this type are quiet after cool down, but suffer from being noisy during cool down from ambient to cryogenic temperatures. This increased noise of operation during cool down is believed to be due to the same over driving of the expander and drive piston which applies to the pneumatic-piston types of expanders. Again, the piston and expander are believed to impact with the end walls of the expander portion of the cooler because the spring rates of the centering springs are not sufficient to control the excursions of the expander and piston during start up and cool down.